Films composed of polyesters within the thickness range specified are sufficiently well known. However, a disadvantage of such polyester films is their hydrolysis tendency, especially at temperatures above the glass transition temperature of the particular polyester. In this context, the hydrolysis tendency is understood to mean the property of the polyester of being degraded hydrolytically under moist conditions, which is noticeable, for example, by a reduction in the IV or SV. This is a limiting factor for the use of polyester films especially in applications with relatively high thermal stress, such as in film capacitors, cable sheathing, ribbon cables, engine protection films, but also in long-term applications such as in glazing and outdoor applications, and especially in the backside laminate of solar modules.
The hydrolysis tendency is particularly marked in the case of aliphatic polyesters, but also in the case of aromatic polyesters such as PBT and PET. When the hydrolysis tendency of PET becomes too great for the application, it is necessary to revert to the more hydrolysis-stable PEN or even to other polymers, for example polyetherimides or polyimides. However, these are significantly more expensive than PET and are therefore frequently no solution for economic reasons.
It has therefore already been proposed to improve the hydrolysis stability of polyester films through the incorporation of hydrolysis stabilizers.
More hydrolysis-resistant polyester raw materials which are obtained through use of carbodiimides, and fibers and films produced therefrom, are known (U.S. Pat. No. 5,885,709, EP-A-0 838 500, CH-A-621 135). Films which are produced from such raw materials, however, tend both in production and in later use to outgas isocyanates and other mucosa-irritant or harmful by-products and degradation products. This is a much greater problem in flat structures such as films with a large surface area than, for example, in injection moldings.
Hydrolysis stabilizers based on terminal epoxy groups can likewise lead to hydrolysis stabilization and are described, for example, in EP-A-0 292 251 or U.S. Pat. No. 3,657,191. Frequently, the incorporation of such compounds into the polyester matrix is inadequate, which leads to breakoffs in the production process in the case of stretched polyester films. Owing to the poor incorporation, in addition, only a portion of the epoxy functions react with the polyester. The remainder react with each other and lead to gelation of the material used. The reactivity of such epoxy functions with polyesters and with each other in the extrusion is very high, such that the hydrolysis stabilization is based here essentially on an initial molecular weight rise (i.e. the films possess a higher IV or SV from the start and therefore need somewhat longer in order to degrade it hydrolytically than identical films with an initially lower IV/SV), with a secondary effect via a reduction of the carboxyl end groups. True stabilization by means of a stabilizer which is still active in the end product and continues to actively offer hydrolysis stabilization in the lifetime of the end product cannot really be achieved by this method. If anything, this is a chain extension and not true hydrolysis stabilization.
Moreover, known hydrolysis stabilizers such as carbodiimides and other substances, as described in EP-A-0 292 251, have the disadvantage that, owing to their chain-extending action, they lead partially to significant, abrupt molecular weight increases (viscosity rise) in the polymer during extrusion, thus making the extrusion process unstable and difficult to control.
In addition, polyester films with epoxidized vegetable oils as stabilizers are described in EP-A-1 634 914 and EP-A-1 842 871. The toxic degradation products which are typical of carbodiimides do not occur here, the incorporation into the polyester matrix is good given suitable selection of the oils, and there is good hydrolysis stabilization of the films. Disadvantages which occur in application are, however, viscosity variations in production, especially toward lower viscosities. This viscosity reduction is noticeable particularly in the case of extrusion in the melt and leads to varying pressures. These lead to thickness variations of the extruded preliminary film and as a consequence to breakoffs in production.
An increase in the viscosity, especially during the extrusion, could be achieved through the use of chain extenders.
Chain extenders for polyesters are likewise known and are described, for example, in EP-A-1 054 031. This publication describes the use of anhydrides, especially pyromellitic anhydride, as an effective constituent of the inventive formulation.
Chain extenders for PET, which are based on oxazolines or caprolactams, are sold by DSM (the Netherlands) under the brand name ALLINCO®.
Chain extenders with epoxy functions are described in U.S. Pat. No. 6,984,694 among other documents and are commercially available under the brand name JONCRYL® from BASF (Germany).
Polymers with glycidyl end groups, which are likewise suitable in principle as chain extenders for PET, are sold under the brand names EPON® by Hexion (USA) or LOTADER® by Arkema (France).
Even though they are well known as described above, chain extenders have to date not found any significant use in industrial practice in the production of polyester films.